Flatulence From a Popular Swimming Hole

Payal Parekh

I was surprised to find out that lots of methane is bubbling out of my favorite swimming hole. Wohlensee, a small run-of-river hydro reservoir in Switzerland (where I lived for the past three years), emits a whopping 780 metric tonnes of methane a year, according to a recent study from Eawag, the Swiss Federal Institute of Aquatic Science and Technology. This is equivalent to the amount of methane produced annually by 6,800 cows. The study only measured methane released in bubbles from the reservoir surface: Actual emissions may be several time higher due to the degassing  of methane at the dam’s turbines and spillway and in the river immediately downstream.

It has usually been assumed that methane emissions are negligible from dam reservoirs in temperate regions and from run-of-river projects. Run-of-river dams have relatively small reservoirs and because of their small storage capacity, it was thought that water would not be detained in their reservoir for enough time for methane to form. This Eawag study throws both these assumptions into the air. The study also destroys the hydro industry’s claims that reservoirs are only high emitters for their first decade or so after construction – the Wohlensee was built in 1920.

Wohlensee's methane bubbles have a warming impact equivalent to 119 grams of CO2 for every kilowatt-hour generated. This is 10 times higher than emissions for wind power, if calculations take into account emissions during wind-turbine manufacture and installation. The comparison is not a fair one as it does not include the cement and fossil fuel consumption from building the Wohlensee, or the likely initial spike in emissions due to rotting vegetation when the reservoir was first filled. Hydropower doesn't seem so clean after all, now does it?

To learn more, take a look at our newest factsheet, Dirty Hydro: Dams and Greenhouse Gas Emissions.